Spirally wound membrane modules are often employed alone or in combination for the separation of materials by high pressure reverse osmosis, for example, for the production of pure water from brine; or low pressure ultrafiltration, for example, in the dairy field, such as for the separation of dairy solutions. The spiral wound membrane module which fits into a cylindrical outer housing shell comprises a central tube sealed within the shell and having a plurality of holes or slots therein which serve as permeate collection means and one or more membrane leaves wrapped around the central tube. The membrane leaf typically comprises a pair of flat sheet membranes selected to effect the desired separation with the membranes separated by an open porous-type mesh material, such as tricot, as a permeate collection material and which forms a permeate collection channel. The membrane leaves are generally separated by a net-like feed channel spacer which can also serve as the bypass. In operation, a feed solution to be separated is introduced into one end of the cylinder and flows directly axially along the feed channel and bypass spacer, and a concentrate stream is removed from the other end of the shell. The permeate which passes through the membrane sheets flows radially through the permeate collection means toward the central tube, and is removed from the central tube at a permeate outlet. Thus, the feed flow is axial across the membrane leaf surface while the permeate flow is radial toward the central tube.
The membrane envelope or leaves are secured together at the outer edges by an adhesive to form a peripheral radial adhesive zone at each end and an axial adhesive zone toward the outer periphery with the inner periphery in permeate communication with the passageways in the central tube. Spirally wound membrane modules and their construction are well known and are illustrated, for example, in U.S. Pat. No. 3,367,505 and other patents.
A particular spirally wound membrane module for osmosis separations is described in U.S. Pat. No. 4,033,878 issued July 5, 1977, hereby incorporated by reference in its entirety. This patent describes a membrane module wherein flow blocking techniques are employed both in the central tube and also in the individual membrane leaves in order to induce a serpentine-type flow of one stream in the membrane envelope.
Spiral wound membrane modules, particularly low pressure modules used for ultrafiltration, such as in the dairy field for the separation of whey, are subject to a cleaning operation after periods of use, generally by the use of chemical cleaning solutions, such as caustics, detergents, chlorine, or combinations thereof, and thereafter, rinsed with clean water. During such cleaning operation, there is a significant increase in permeate flux due to the removal of the fouling layer on the membrane surface. The high increase in flux during the cleaning operation causes an excessively high pressure within the permeate channel because of the higher flux. The increased pressure within the permeate channel often causes wrinkles to occur in parts of the membrane leaf, and the zone may fail due to embrittlement and cracking under the high pressure and high flux conditions. These wrinkle and leak problems are confined particularly to high flux ultrafiltration spiral membrane modules and not to reverse osmosis membrane modules which usually operate at high pressures of 300 psi or more and low flux. Therefore, it is desirable to provide a spirally wound membrane module for low pressure operation, such as ultrafiltration, which will have a reduced tendency to wrinkle or leak in operation or particularly during a cleaning cycle of the spirally wound membrane module and to a method of manufacturing and using such modules.